1. You do not need to know any of the terminal commands to work with Linux. None.
2. If you do not have access to your graphical user interface, you can still use your computer.
Here we aim to dispel the myth that "Linux is a text-based operating system."
First of all, we need to understand soundly what a terminal is and what a window manager is. The terminal (command line, command line interface, CLI, console, whichever you prefer) is basically a prompt that waits for the user's commands. Upon receiving a request for execution, it passes the commands to the operating system and then returns the results. The graphical user interface (GUI) is the screen and its decorations where you see the graphical user elements such as windows, menus, and icons.
In Microsoft Windows, both a GUI and a CLI exist. You can do many administrative tasks via the command line, even though the commands themselves are pretty much hidden. In Windows the GUI and the CLI are integrated: you cannot close the GUI and leave yourself with the CLI (except for in some very rare or administrative instances).
In Linux, the GUI and CLI are very closely integrated as well as infinitely separable. As I described above, moving a file to another location is basically executing some command in the background. You can do exactly the same thing via the CLI. They are also separated, meaning that you can close the whole GUI and work from the CLI. Fedora, for example, does not install a GUI by default if you choose "server installation." (But you can later install one yourself.) You can also set up your server so that it does not show you the GUI, but the other clients connected to it can work with the graphical interface.
The point here is Linux's GUI is just a server that works like any other program. In the same way that a web server displays web pages to you, the graphical server displays the graphical user elements to you. That’s why it is called “X Server."
Doing things this way has its conveniences, too. If you do happen to crash the GUI in your Linux box (which is a hard task), simply switch to the command prompt, shut down the GUI, restart it, and continue working. You are not left with a completely frozen system. Imagine having Microsoft Word frozen. You can close it, then restart it, and continue working. However, if Windows itself crashes, Ctrl-Alt-Del and the reset button are your only options.
In Windows, the GUI is the same for a particular Windows release, for every default installation. Almost everybody knows the green "Start" menu button with blue taskbar, and the grassy hill with the blue sky in Windows XP. This is not the same in Linux. The user is not forced to use one GUI. Instead, the user is free to choose any GUI that he or she wants. Some of these include KDE, Gnome, Fluxbox, Enlightenment, and XFCE.
The user is also not limited to a certain set of applications. These graphical user interfaces, or their desktop environments to be more precise, can run almost all of your applications. You can choose to work in a Gnome environment, but choose to use Kontact, KDE's native productivity application, rather than Evolution, which is Gnome's native application. You can choose to work with one of the most lightweight desktop environments, such as Fluxbox, but continue to work without interruption with LibreOffice. You can choose XFCE as your desktop and surf with Firefox, which uses Gnome libraries for its graphics.
The graphical user interface may be confusing for newcomers to Linux, but this is another point where the operating system shines: nothing is dictated to you. You can choose more eye candy, more resources, and more elements if your system is powerful, just as you can choose less or no eye candy, fewer resources, and fewer elements if your system is not powerful enough. You can even choose to run no GUI at all if you have a veteran PC like a Pentium II/200 MHz. This is not a flowery explanation. If curious, you can check out the minimum requirements for the Slackware Linux distribution here.
If you look at to the desktop environments one by one, you will better understand what we mean by "freedom of choice," "eye candy," and "low resources."